1. Field of the Invention
This invention relates to a magnetic thin film memory device designed to record or reproduce information in accordance with the direction of magnetization.
2. Description of Related Art
FIG. 1 is a diagram of a conventional magnetic thin film memory device disclosed in "Magnetic Thin Film Engineering" (p. 254, Magnetic Engineering Lecture 5; Maruzen Co., Ltd., 1977).
An example how to manufacture the memory element will be discussed in the first place. A mask with rectangular holes is brought in tight contact with a smooth glass substrate G, onto which a vacuum deposited film of Fe, Ni about 2000 .ANG. thick is formed within a vacuum apparatus. As a consequence, many magnetic thin film memory elements MF are manufactured in matrix at one time. A driving line to drive the magnetic thin film memory elements is obtained by photoetching copper strips on both surfaces of a thin epoxy resin plate or a thin polyester sheet in a manner that the strips on the one surface to be orthogonal to those on the other surface. The lines on the both surfaces are rendered word lines and digit lines, respectively, and the memory device is assembled in a manner that each crossing point of the lines is arranged to overlapped onto each memory element.
The principle of the operating of the memory element will be depicted. The lines parallel to the axis of easy magnetization in the drawing are word lines W1 through W3, while those orthogonal to the axis of easy magnetization are digit lines D1 through D3. The digit line serves also as a sense line to read the storing state of information in the memory element. The magnetization in the film is stabilized along the axis of easy magnetization corresponding to the storing state of information "0" or "1" in the memory element. Specifically, a white upward arrow in the drawing shows that information "0" is stored and a white downward arrow shows that information "1" is stored in the memory element. Supposing that magnetic fields acting to the magnetic thin film by a digit current Id and a word current Iw are respectively Hd and Hw, when the current Iw of a unipolar pulse is allowed to run by selecting the word line W1, the magnetic field Hw acts to the whole of the memory elements MF below the word line W1, and the magnetization is directed to the axis of hard magnetization. At this time, pulse voltages of the opposite polarities are induced to the digit lines D1 through D3 which become reading voltages depending on whether the magnetization is turned from the "1" state or "0" state. In recording, the digit current Id is fed as to overlap the trailing edge of the Iw pulse, and in the condition of the magnetization being directed in the axis of hard magnetization, the magnetic field Hd of the polarity corresponding to an information signal is super-imposed, thereby determining the direction of magnetization in order to record information in the "1" state or "0" state. The value of Iw is set to generate the magnetic field Hw sufficient to turn the magnetization of the magnetic thin film from the axis of easy magnetization to the axis of hard magnetization. The value of Id is set to generate the magnetic field Hd about half the coercive force Hc of the magnetic thin film.
Among the memory elements MF along the word line W1, the upper one reads "1" and writes "0", the middle one reads "0" and writes "0" and writes "1", and the lower one reads "0" and rewrites "0" after reading. As is clear from the foregoing description, since the magnetization immediately after reading is directed to the axis of hard magnetization, and it is infinite to which direction, "1" or "0", the magnetization is turned, the turning direction is determined by applying the magnetic field Hd.
In conventional reading method, since a minute electromagnetic induced voltage resulting from the rotation of the magnetization is used. Therefore, the S/N ratio at reading is so small that read-out was difficult. Moreover, since the electromagnetic induced voltage is proportional to the size of the magnetic moment, it is required to make the magnetic thin film larger to obtain a large electromagnetic induced voltage. In consequence, the magnetic field necessary for recording/reproducing is undesirably enlarged, thereby causing a hindrance to saving of power. The amount of information stored per unit area is impossible to be increased.
Meanwhile, a magnetic thin film memory device which reads information with use of the magnetoresistance effect is already known. FIG. 2 shows the principle of a magneto-resistive element disclosed in "Magnetic head and magnetic recording" (pp. 182-190, M. Matsumoto; Sogo Denshi Shuppan). In FIG. 2, a reference numeral 101 indicates a magneto-resistive element formed of a magnetized film with the axis of easy magnetization denoted by A. The magnetoresistance effect is a phenomenon that when a current I runs in the magnetoresistive element 101 to impress an external magnetic field H, thereby to change the direction of magnetization, resistance of the magnetoresistive element is changed by an angle of the direction of the current I to the direction of magnetization M corresponding to the external magnetic field H.
FIGS. 3 and 4 are a perspective view of a conventional magnetic thin film memory element and a circuit diagram of a magnetic thin film memory device using the conventional element revealed in "Reprogrammable Logic Array Using M-R Elements" (pp. 2828-2830, IEEE Transactions on Magnetics, Vol. 26, No. 5; Sep., 1990). In FIGS. 3 and 4, reference numerals represent respectively: 101a, 101b a magnetic thin film of permalloy or the like having the magnetoresistance effect: 102 a metallic thin film of copper, etc. sandwiched between the magnetic thin films 101a and 101b; 103 a word line for applying an external magnetic field to the magnetic thin films 101a, 101b; 111 a magnetic thin film memory element; 112 a sense line constituted of the magnetic thin films 101a, 101b and metallic thin film 102; 113 a dummy line corresponding to the sense line 112; 114 a switching element for determining the direction of a voltage to be fed to the sense line 112; 115 an autozero circuit for detecting a zero signal automatically; 116 a differential amplifier; 117 a switching element for determining the sense line 112 to be accessed; and 125 a comparative resistance on the dummy line 113. The word line 103 is formed orthogonal to a current running in the magnetic thin films 101a, 101b, and parallel to the axis of easy magnetization A of the magnetic thin films 101a, 101b.
The above magnetic thin film memory device operates in a manner as follows. First of all, the magnetoresistance effect will be explained below. As shown in FIG. 5, an external magnetic field Hex is applied in the direction of the axis of hard magnetization 151 so as to direct the magnetization 152 of the magnetic thin film 101 at an angle .theta. to the direction of the axis of easy magnetization 150. At this time, by impressing a voltage E to both ends of the magnetic thin film 101 and measuring a sensor current (i) by an ammeter 162, the relation between the direction of magnetization and current (i) becomes as indicated in a graph of FIG. 6. In other words, when the direction of magnetization 152 is parallel to the running direction of the current (here, direction along the axis of hard magnetization 151) (.theta.=.+-.90.degree.), resistance of the magnetic thin film 101 becomes maximum. On the other hand, when the direction of magnetization 152 is perpendicular to the direction of current (.theta.=0.degree.), resistance becomes minimum (the current flows most).
The operation of the magnetic thin film memory element illustrated in FIG. 3 will be described now. In recording, when a word current is allowed to run in the direction shown by an arrow, the direction of the magnetic field generated by the current is the direction of the axis of that magnetization 151 of the magnetic thin film 101, thereby to turn the direction of magnetization 152 (referring to FIG. 5) to be the direction of the axis of hard magnetization 151, if a sufficient amount of current is allowed to flow. Subsequently, a current is supplied to the sense line 112 to determine the direction of magnetization. Although the magnetic field generated by this current is reverse in direction between the magnetic thin films 101a and 101b, both are along the axis of easy magnetization 150. Therefore, the direction of magnetization can be determined by shutting off the current of the word line 103. As indicated in FIG. 7, the directions 152a, 152b of magnetization of the magnetic thin films 101a, 101b are determined by the direction of a current 163 running in the sense line 112.
Next, in reproducing, it will be discussed in conjunction with only the magnetic thin film 101a for brevity's sake with reference to FIG. 8 which is a bottom view when a smaller current than in recording is allowed to flow in the word line 103. The direction of magnetization 152a is inclined .theta., to the direction of the axis of easy magnetization 150 because of the magnetic field Hex generated by the current running in the word line 103. This fact holds true both in FIGS. 8A and 8B except that the angle .theta., is plus or minus. Then, when the current 163 is fed to the sense line 112 a magnetic field Hsf is generated by the sense line 112 as shown in FIG. 9. The direction of magnetization 152a is determined by the external magnetic field 60. An angle .theta..sub.2 of the direction of magnetization 152a to the direction of the axis of easy magnetization 150 is varied in accordance with the recorded state of magnetization, and the recorded state of magnetization can be detected as an increase or decrease of the electric resistance as shown in FIG. 6.
The operation of the memory device will be discussed with reference to FIG. 4. In recording, only the memory element 111 having both the word line 103 and the sense line 112 simultaneously turned on is driven. Although the recording state is determined by the direction of the current running in the sense line 112 as described before, the direction of the current is decided by the switching element.
In reproducing, firstly, by turning on the switching element 117 of the sense line 112 to be accessed without supplying a current to the word line 103, the potential at a connecting point x is compared with that at a connecting point Z of the dummy line 113 by the auto zero circuit 115 and the potential difference is stored therein. Thereafter, by supplying a current to the word line 103, the recording state of the element is detected according to whether the potential difference becomes larger or smaller than the stored difference.
As described before, since the memory elements 111 in the conventional magnetic thin film memory device are connected in series to the sense line 112, resistances of the memory elements 111 work in series, making the impedance of the sense line 112 larger in proportion to the number of the connecting memory elements 111. As a result, only a limited number (4 in the prior art) of memory elements can be arranged on a single sense line so as to secure the sufficient S/N ratio. Fundamentally, the signal is detected by the static resistance of the sense line 112, thus requiring the comparative resistance 125. The temperature compensation of the resistance of each memory element 111 becomes necessary, therefore the comparative resistance 125 must be formed of a magnetic thin film. As such, the prior art is disadvantageous in its complicated structure with little allowance of design.
Moreover, ferromagnetic alloy made of Ni, Fe, Co or the like used conventionally as a magnetoresistive element for read-out is magnetized thin film having horizontal magnetic anisotropy. A large external magnetic field is needed against a demagnetizing field in order to change the magnetization M in a perpendicular direction to the film surface. Therefore, the conventional magnetoresistive element is poorly low in detecting sensitivity to the magnetic field perpendicular to the film surface.